Method and device for synchronization in wireless networks

ABSTRACT

A method and device are provided for synchronizing data transmission of multicasting/broadcasting services (MBS) by a plurality of Base Stations. The method disclosed includes providing synchronization information to enable the plurality of Base Stations to start transmitting MBS data at a synchronized starting time point. Meanwhile, each of the Base Stations receives the MBS data to be transmitted and determines whether any of the MBS data has not been properly received. If so, the respective Base Station initiates a process to recover the missing MBS data or to obtain within a pre-determined time interval, information regarding the missing data to determine the duration of the time period that would have been required for transmitting the missing MBS. If the missing data has not been timely recovered, the respective Base Station determines a starting point and duration of a silence period based on the information obtained, and refrains from transmitting signals along a communication channel allocated for transmission of MBS data, during that silence period.

FIELD OF THE INVENTION

The present invention relates to method and apparatus adapted to improvetransmission efficiency in wireless communications systems, and moreparticularly in wireless communications networks that supportmulticast/broadcast services.

BACKGROUND OF THE INVENTION

Multicast/Broadcast Service (“MBS”) in WiMAX Networks is a service thatallows the distribution of data to a group of Mobile Subscribers(“MSs”). IEEE 802.16e standard introduces the notion of MBS_Zone—an areain which multiple Base Stations (“BSs”) synchronously broadcast the samedata over the same subchannels at the same time. This technique greatlyimproves the mobile terminals ability to receive data correctly due toincreased energy of the combined signal that each mobile terminalreceived simultaneously from multiple Base Stations.

MBS Service-Flow (“SF”) carries information to a set of MSs. Typicallythere are two methods to access a group of MSs for the provisioning ofMBS:

-   -   Single-BS: Transmission of data over a single Base Station        (“BS”) in the network. The SF is mapped to a Connection        Identifier (“CID”) within a specific BS, i.e., the CID is        uniquely specified on a “per BS basis”.    -   Multi-BS: Transmission of data over a plurality of BSs in the        network in a synchronized manner. The SF is mapped to a CID        unique within a zone at which the service is provided, referred        to hereinafter as an MBS_ZONE.

The establishment of an MBS connection is typically carried out in a waysimilar to the way by which unicast connections are established, whilethe MS registers to the network. This service, the MBS, is maintainedregardless of the current mode of the MS (Normal/Sleep/Idle), so thatMBS data is transmitted and received regardless of the MS currentoperation mode.

The Multi-BS access method enables an MS to receive the MBS content,after having successfully registered and the connection established,from several BSs. As explained above, this transmission method requiresthat the group of BSs participating in the same Multi-ES-MBS service tobe synchronized so that data shall be transmitted by all these BSssimultaneously, and to use the same CID and Security Association (“SA”).It should be noted that the MS does not have to be registered at thespecific BS from which it receives MBS transmissions.

An MBS_ZONE identifier is used to indicate the group of BSs which usethe same CID and SA to distribute an MBS SF. MBS_ZONE can be advertisedby the BS in DCD messages, also it can be delivered upon establishmentof MBS connection and it can be extracted from the MAP_MBS_IE.

Obviously an MBS_ZONE may include one or more BSs, and a BS may havemultiple MBS_ZONE identifiers.

In order to achieve the necessary level of synchronization and allow PHYdiversity the downlink data should be identically mapped onto theairburst subchannel-time continuum, a coherent MBS_MAP should becreated, and IEEE-802.16e Generic MAC headers, Fragmentation Subheaders,and Packing Subheaders should be applied identically across all the BSsthat belong to the same MBS_Zone (see FIG. 1).

However, one of the major drawbacks of the currently known systems isthe need to maintain synchronization of MBS downlink (“DL”) flowsbetween the BSs at a level that enables PHY diversity over WiMAX radiolinks, while the data itself is generated and conveyed alongnon-synchronized packet networks having high transmission jitter.Furthermore, as no synchronization-related information can be added tothese packets, they cannot be used as a source for synchronizationinformation.

One other problem associated with the implementation of the MBS-Zoneconcept, lies in the fact that each MBS_Zone should be synchronizedindependently of the other MBS_Zones. However, in case of overlappingMBS_Zones, i.e., a BS that is a member of several MBS_Zones, there is aproblem of mutual interference (meaning transmission of different dataover the same sub-channels at the same time) which must be avoided. Onthe other hand, if two Zones do not overlap, it would be beneficial toallow them to transmit different data over the same sub-channels at thesame time in order to improve resources' utilization. Thus, a mechanismfor assigning time and subcarrier regions for each MBS Zone is required,where such a mechanism takes into account the geographical distributionof the zones, together with the presence or absence of actual data fortransmission. The importance of the latter is because when a certainzone has no data to transmit at a certain given time, it will notproduce interference and its resources (time and sub-channels) can bereused.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus that allow efficient utilization of the bandwidth available inwireless networks.

It is another object of the present invention to enable efficientmulticast synchronization across arbitrary large MBS zones, preferablyby maintaining separation between the radio and networkingintelligences.

It is yet another object of the present invention to provide a methodand device for synchronization of MBS DL flows between BSs to enable PHYdiversity over WiMAX radio links.

It is still another object of the present invention to allow efficientmulticast synchronization between overlapping MBS Zones.

Other objects of the invention will become apparent as the descriptionof the invention proceeds.

Thus, in accordance with the present invention there is provided in awireless communications network comprising a plurality of Base Stationseach operative to provide MBS to mobile subscribers, a method forsynchronizing the provisioning of MBS data by the plurality of BaseStations comprising the steps of:

(i) providing a first pre-determined time interval;

(ii) providing synchronization information to enable all of theplurality of Base Stations to start transmitting MBS data at asynchronized starting time point;

at each of the plurality of Base Stations:

-   -   (iii) receiving MBS data to be transmitted by all of the        plurality of Base Stations;    -   (iv) determining whether any of the MBS data has not been        properly received (e.g. not missing, corrupted, etc.), and upon        determining the existence of such faulty MBS data (i.e. that has        not been properly received), initiating by the respective one or        more Base Stations a process to recover the faulty MBS data or        to obtain within the first time interval information regarding        the faulty data for use in determining the duration of a time        period that would have been required for transmission of the        faulty MBS data;    -   (v) for any of the respective one or more Base Stations that has        not recovered the faulty MBS, determining a starting point of a        silence period which would proceed the data that has been        properly received at the respective Base Station and the        duration of said silence period based on the information        obtained by that respective Base Station;    -   (vi) transmitting available MBS data, starting at the        synchronized starting time point and for the duration of a        pre-determined second time interval, and wherein each of the        respective one or more Base Stations that has not recovered the        faulty MBS data shall refrain during said silence period from        transmitting any signals along a communication channel allocated        for transmission of MBS data.

By a preferred embodiment of the invention the duration of the firstpre-determined time interval is equal substantially to twice theduration of the second time interval. According to another preferredembodiment of the invention, steps (iii) to (vi) are repeated everyinterval of time that is equal to three times the duration of the secondtime interval.

In accordance with another preferred embodiment of the invention, steps(ii) to (vi) are repeated after receiving synchronization informationfrom a network synchronization/scheduling means.

By yet another preferred embodiment, the synchronization informationcomprises at least one of the following:

-   -   Global multicast flow identification for identifying a        respective multicast flow;    -   Timestamp indicating the start of the T_(1.) interval; and    -   Sequence numbers of the first and last packets received during        the first pre-determined time interval;

According to another embodiment, in addition to the above, thesynchronization information for non-homogenous scheduling furthercomprises information that relates to at least one of the following:

-   -   allocation of the MBS Bursts across the air frames of the        transmission period;    -   Order of bursts;    -   Information allowing the BSs to build identical PDUs (e.g.,        fragmentation, packing, first FSN, etc.).

According to still another preferred embodiment of the invention, theprocess in step (iv) to recover the faulty MBS data or to obtaininformation about the time period required for transmission of thatfaulty MBS data, comprises the steps of:

(a) sending by each of the respective one or more Base Stations acorresponding recovery request message to a distributing gateway whichcomprises identification of the respective multicast flow and a list ofsequence numbers of faulty packets;

(b) receiving from the distributing gateway a recovery response messagecomprising identification of the respective multicast flow and a list ofsequence numbers of the faulty packets together with informationrelating to their corresponding lengths.

In addition, the recovery response message may further compriseinformation relating to the data included in one or more of the faultypackets.

By yet another preferred embodiment of the invention, step (vi) issynchronized with a paging period of respective mobile subscribers sothat their mobile terminals can avoid waking up twice, once to receiveMBS data and the other time to receive paged data, during two differenttime intervals.

According to another preferred embodiment of the invention, the wirelesscommunications network comprises a plurality of MBS zones, eachcomprising at least one Base Station, wherein at least two of the MBSzones geographically overlap each other, and wherein the method providedfurther comprises a step of synchronizing all MBS zones that have nooverlapping between each other so that they may transmit their data overthe same subcarriers at the same time.

Optionally, the step of synchronizing all MBS zones is carried outdynamically so that after each first interval period, the networkresources are re-assigned and distributed among all Base Stations thatbelong to the corresponding MBS zones.

In accordance with another aspect of the invention there is provide asynchronization device adapted to operate in a wireless communicationsnetwork comprising a plurality of Base Stations each operative toprovide MBS to mobile subscribers, comprising:

a clock synchronized with clocks of the plurality of Base Stations;

a processor adapted to carry out one or more scheduling algorithms andoptionally burst construction logic;

a receiver adapted to receive MBS data;

a buffer operative to store MBS data received; and

a transmitter operative to transmit synchronization information to theplurality of Base Stations.

Preferably, the synchronization information comprises at least one ofthe following:

-   -   Global multicast flow identification for identifying a        respective multicast flow;    -   Timestamp indicating the start of a time interval during which        MBS data shall be accumulated by the plurality of Base Stations        and each of the Base Stations may conduct a recovery process in        case some or all of the MBS data received thereby is missing;        and    -   Sequence numbers of the first and last packets received during        that time interval.

Preferably, the synchronization information further comprisesinformation that enables coherent construction and placement of bursts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1—Mapping MBS transmission buffer into an IEEE 802.16e MBS burst

FIG. 2—presents a schematic view of synchronization periods according toan embodiment of the present invention;

FIG. 3—presents a schematic illustration of a network architecture thatincludes a distributor gateway;

FIG. 4—shows a schematic illustration of network architecture withreliable multicast infrastructure;

FIG. 5—shows an example of MBS bursts allocations for accumulated data;

FIG. 6—presents a schematic view of multicasting synchronizationinformation;

FIGS. 7, 7A and 7B—show schematic representations of a multi MBS zonesnetwork with partial overlapping;

FIG. 8—illustrates use of coloring method for scheduling MBStransmission in the network of FIG. 7;

FIG. 9—illustrates transmissions of several MBS zones within a singleframe;

FIG. 10—illustrates transmission of a single MBS zone within a frame;and

FIG. 11—illustrates the architecture of a single sync node for thenetwork of FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A better understanding of the present invention is obtained when thefollowing non-limiting detailed description is considered in conjunctionwith the following drawings.

For the sake of simplifying the embodiments described hereinafter, thefollowing assumptions are used.

-   -   1. MBS is essentially a unidirectional service and thus is        insensitive to delays. Some level of feedback could be        associated with such a service (e.g. interactive TV) but there        would be no adverse effect if the feedback is delayed for a        couple of seconds or so. Thus the synchronization mechanism may        introduce a few seconds of delay. The delay is recommended in        order to compensate for the high transmission jitter that may        occur in a WiMAX backhaul network.    -   2. The physical clocks of all Base Stations in the network are        synchronized. The nature of synchronization is out of scope of        this invention, and can be done in any one of the well-known        techniques per se to synchronize the clocks across a network.        For example GPS or IEEE 1588 may be used.    -   3. The nature of traffic is arbitrary so that the size of a data        burst as well as time intervals between two consecutive bursts        cannot be predicted.        Synchronization Process

The principles of synchronization are illustrated in FIG. 2, where thetime axis is divided into periods having duration of τ; while τ is aproduct of an integer multiplied by an airframe duration. Let T₀represent the starting moment of a period of duration τ. Then data thathas been accumulated during the period extending from T₀ to T₀+τ (alsoreferred to as accumulation period) will be transmitted during theperiod extending from T₀+2τ to T₀+3τ (also referred to as transmissionperiod). During the period extending from T₀+τ to T₀+2τ (which isreferred to as a recovery period) missing data (or at least the lengthof the missing data) must be recovered.

On the other hand, as the recovery and accumulation periods may overlap(due to the asynchronous nature of data distribution in the backhaulnetwork) time can be regarded as comprising two main periods:distribution of data (e.g., accumulation and recovery) between T₀ toT₀+2τ and transmission between T₀+2τ to T₀+3τ.

The recovery of missing data (or at least the length of the missingdata) is essential because otherwise it will be rather difficult toensure that all the BSs transmit the same data at the same time over thesame subcarriers. If recovery of the missing data is impossible then atleast the lengths must be recovered. In the latter case, where only thelength of the missing data is available to the corresponding BS, it willtransmit nothing in the relevant subcarriers at the time when themissing data should have been transmitted for a duration determined bythe recovered length of the missing data.

Obviously this method introduces a delay up to 3τ. Preferably, the valueof τ is substantially larger than the longest round trip delay expectedin the network, in order to allow enough time for lost data recoveryduring the recovery period. So if the longest round trip delay in thenetwork is about 100 msec, τ can be equal to 200 or 300 msec. In thiscase the delay introduced by the synchronization will reach 600 or 900msec, respectively.

In addition, in order to increase the power-saving of MSs in Idle-Mode(“IM”), the transmission period may be synchronizes with the IM pagingcycle. In this case, power will be saved as the MS will not be requiredto wake up and receive data during two different time intervals (MBSdata transmission, and MOB_PAG-ADV transmission). In order tosynchronize the paging cycle with the MBS distribution cycle, thefollowing should preferably be satisfied: τ should be τ>2 frames andα×2τ<PAGING_CYCLE<α×3τ (α=1, 2, 3, . . . ).

The data distribution method is flexible and can support the Single-ESimplementation. In this case an MBS zone would include a single BS, andwould not require synchronization. Thus, τ can be set to zero and the BSbe allowed to schedule transmission of data as soon as it is received.

Assigning Sequence Numbers to Multicast Packets

According to the WiMAX NWG Architecture the ASN GW communicates with BSsvia R6 data and control plane protocols. This architectural framework isdefined only for unicast traffic distribution. The unicast traffic isforwarded from the Anchor ASN GW to the corresponding Base Stations overGRE tunnels. The Anchor ASN GW classifies the user data and maps themonto the corresponding GRE keys. The BS then maps data tagged with acertain GRE key onto the corresponding IEEE 802.16e Connection.

According to an embodiment of the present invention, the above-describedarchitectural framework is reused by defining that the multicast dataare distributed from the Distributor GW to the corresponding BSs overGRE tunnels. The architecture is shown in FIG. 3. The Distributor GWclassifies the flows by Source and Destination addresses and maps thematching data onto corresponding GRE key. The GRE Tunnel may be per BS(unicast) or per group of BSs (multicast). The BS would then map eachsuch GRE Tunnel onto M-Cast IEEE 802.16e connections.

In addition to classifying the multicast data and mapping them onto GREtunnels, the Distributor GW assigns GRE sequence numbers to the datapackets it forwards to the BS. The sequencing is carried out per GREkey, thus a GRE Key and a GRE Sequence Number uniquely identify thecorresponding data packet.

The multicast routing infrastructure above the Distributor GWs maysupport Reliable Multicast Delivery mechanisms (e.g. PGM described inRFC 3208, SRMP described in RFC 4410, NORM described in RFC 3940, andthe like). These protocols also deploy sequence numbering in one form oranother which are assigned by the traffic source. These sequence numbersmight be copied into the GRE Sequence Numbers of the corresponding datapackets distributed from the Distributor GW to the BSs. Thus globalsynchronization of sequence numbers will be achieved within the entiremulticast distribution tree rooted at a particular traffic source.Consequently, it becomes possible to deploy multiple Distributor GWacross the same MBS zone as it is shown in FIG. 3.

Distributing Synchronization Info

Synchronization information is multicast to the entire set of the BSs inthe MBS_Zone, every period of time having the duration of τ, by apre-determined entity in the network. This entity that generatessynchronization information frames is referred to herein as a “SyncNode” (see FIG. 4). The Sync Node is preferably a Base Station, so thatits clock is synchronized with the clocks of other BSs in the MBS zone,it runs BS scheduling algorithms, and receives the same data as does anyother BS in the MBS_zone. It is not important if the Sync Node actuallytransmits the multicast data over the air or not, as the relevant roleit performs in accordance with the present invention is as scheduler(synchronizer) and optionally as bursts constructor for the other BaseStations.

Each synchronization information (“Sync Info”) packet preferablycontains at least one of the following data, and more preferably acombination of them all:

-   -   Global Multicast Flow Identification. Uniquely identifies the        multicast flow in the network. One of options for Global        Multicast Flow ID may be for example, a set consisting of        Multicast Destination IP Address and a list of Unicast Source IP        Addresses, similar to the way by which a multicast group is        represented in IP Routers.    -   Timestamp with granularity ≦τ. The timestamp refers to the start        of the accumulation period (as depicted in FIG. 2).    -   Sequence numbers of the first and last packets received during        the accumulation period.

If all the Base Stations within the MBS_Zone run the same schedulingalgorithms and have received or recovered all the packets the SequenceNumbers of which are specified in the Sync Info message, then thetransmission may be synchronized across the entire MBS Zone using onlythe information referred to above.

In case of distributed scheduling the Sync Info should preferablyinclude additional data that will enable the BSs within the MBS_Zone toschedule and transmit the same data at the same location in the DLframe. That can be done for example by using one of the following twoapproaches:

-   -   A semi-synchronized approach where all BSs have a set of        schedulers; and    -   A full-unsynchronized approach where there is no a priori        knowledge on the schedulers implemented in the BSs.

In the semi-synchronized approach, each of the BSs holds the same set ofschedulers. Therefore, the synchronization information should enable theparticipating BSs to be aware of the fragmentation and packing rules,which scheduler they should use, and if needed with which scheduler'sparameters. For example, in case of Round-Robin scheduling, theadditional information can include the order of connections, and thedeficit counter, etc.

In the full-unsynchronized approach, a BS does not have any a prioriknowledge on the schedulers implemented in other BSs. Therefore thesynchronizing node should distribute the information that will enablethe participating BSs to schedule the connection in a synchronizedmanner.

The synchronizing node should preferably distribute the followinginformation:

-   -   The allocation of the MBS Bursts across the air frames of the        Transmission Period (see FIG. 5) Order of bursts;    -   Information allowing the BSs to build identical PDUs (e.g.,        fragmentation, packing, first FSN, etc.).

Let us take now an example where the Sync Node distributes the MBS_MAPand the relevant IEs. The distribution of data can either be throughdedicated messages or piggybacked with the data. For example, in theaforementioned example, the MAP can be sent in a single message and theIEs can be piggybacked with the data of the relevant connection.

The overheads associated with the synchronization can be reduced andmoreover the method can be further improved by using the information ina periodic manner. In other words, the same MBS_Zone structure in the DLframe will be used in future MBS transmissions in a certain pattern. Forthis purpose the information should preferably also include:

-   -   Number of frames between transmissions (of the same structure);        and    -   Number of times the same MBS Zone is transmitted (see FIG. 5).

When the transmission of the MBS_Zone is completed, the BSs start usingthe new synchronization information.

Lost Content Recovery

In order to achieve transmission synchronization, the data available atevery BS participating in the MBS at the beginning of the transmissionperiod must be identical. If some BSs miss some data then as explainedabove the BSs must recover either the data themselves or, at least, thelengths of the missing packets. A BS may detect data loss by examiningthe sequence numbers in the contiguous stream of packets having the sameGRE Key and by comparing the received sequence numbers with thosespecified in the Sync Info packet.

Next, once the BS establishes that certain data has been lost, in orderto recover that lost data within the realm of a single Distributor GWthe following method may be used:

-   -   A BS that discovers data loss sends a Recovery Request message        to the Distributor GW. The message contains the GRE Key        associated with the Multicast Flow (or alternatively the Global        Multicast Flow Identifier) and a list of Sequence Numbers of the        missing packets.    -   The Distributor GW responds to the Recovery request message by        sending a Recovery Response message. The latter message contains        the GRE Key associated with the Multicast Flow (or alternatively        the Global Multicast Flow Identifier) and a list of Sequence        Numbers of the missing packets together with their corresponding        lengths. Optionally or in the alternative, the packets        themselves are retransmitted over the unicast or multicast GRE        tunnels associated with the Multicast Flow.

The above described recovery mechanism may be augmented by several othermethods to recover data:

-   -   Additional encoding to recover lost and damaged packets;    -   Additional data (e.g., sequence and size) on consecutive packets        attached to every packet distributed in the Mcast tree; and    -   Distribution of metadata (e.g., sequence and size) over the        Mcast tree or another tree dedicated for recovery using        standalone packets.        Synchronizing of Overlapping MBS Zones

In accordance with another embodiment of the present invention, there isprovided a solution to the problem of synchronizing MBS_Zones which mayoverlap with each other to some extent or another.

Let us consider FIG. 7 which demonstrates a system having some overlapin the MBS_Zones associated therewith, whereby each of the zones 1, 2, 4and 5 partially overlap zone 3, and 1 and 2 also partially overlap eachother. FIG. 7A presents schematically the geographical overlapping ofthe zones, whereas FIG. 7B shows a graphical representation of thesezones as the first step of the solution suggested.

Graph vertex coloring algorithm has been selected as the proposedmechanism, and the following example is used to demonstrate how thealgorithm can be used to solve the problem of Inter MBS ZoneSynchronization.

A set of overlapping MBS Zones may be modeled as a graph in whichvertices represent the MBS Zones, and edges defines the relationsbetween the vertices (zones). If two MBS Zones overlap geographically,an edge is drawn between the corresponding vertices. In order to colorthe graph, i.e., creating a vertex colored graph, different colors areassigned to the vertices such that no two adjacent vertices share thesame color. Such a resulting graph with different notations designatingdifferent colors is illustrated on 7B.

Using Graph Coloring for Scheduling

The color assigned to each zone is interpreted as follows: all MBS Zonesto which the same color has been assigned, may transmit their data overthe same sub-channels the same time. In other words, the transmissioncan overlap in both the subchannel and timeslot dimensions, while MBSZones that have been assigned different colors must not transmit theirdata over the same subcarriers at the same time.

Practically, it means that each color is assigned time×sub-channelsregions in which the related MBS Zones (the ones having the same color)may transmit their data. FIG. 8 shows an example of such arrangement. Inthis example, MBS Zones 1, 4 and 5 transmit in one of the regions, MBSZone 2 transmits in another regions while MBS Zone 5 transmits in yetanother region. The MBS Zones topology and the corresponding coloredgraph are depicted in FIGS. 7A and 7B. FIG. 10 depicts the transmissionof multicast connections of only one of the MBS zones within a frame.Obviously, in this example, the same frequency and time resources may beutilized in zones 4 and 5 (the zones that do not overlap with zone 1).

FIG. 8 illustrates a scenario where each airframe is assigned to exactlyone single colored region. In the general case some airframes may haveno colored regions at all (which means no MBS transmission has beenscheduled for these frames). In addition, it is also possible to havemore than one colored region in the same airframe.

FIG. 9 illustrates the transmission of multicast connections of threeMBS zones in a single frame. In this case the transmissions are totallyseparated (either in the frequency domain or time wise) to avoidinterference. In this example, the same frequency and time resources maybe utilized in zones 4 and 5 as in zone 1.

Off-Line (Static) Graph Coloring

Vertex coloring of a graph that models the MBS Zones topology may becalculated off-line and the resulting colored regions may be manuallyconfigured in the corresponding Base Stations. In this case the radioresources for each MBS Zone are pre-assigned. However if at a particulartransmission period, a certain MBS Zone may not have enough traffic inorder to fill the colored region assigned to it, in which case it mayuse the remaining resource (i.e. time and subcarriers) for transmittingnon-MBS traffic.

This approach would not require exchanging any information across thenetwork while still allowing dynamic resource sharing between the MBSand non-MBS traffic within a specific MBS Zone.

On-Line (Dynamic) Graph Coloring

If the radio resources should be dynamically shared between the MBSZones for MBS traffic, then the vertex coloring must be dynamicallyre-calculated so that after each accumulation period and the assignmentof resources is re-assigned and the resulting assignment of the coloredregions is distributed among all the Base Stations that belong to thecorresponding MBS Zones.

In addition to the advantage that can be achieved by using staticcoloring, i.e. sharing resources with non-MBS connections when there isno traffic, dynamic coloring also allows resource sharing amongdifferent MBS zones.

The drawback of this approach is by the complexity of the graph to becolored. If the graph is too complex then the time required for carryingout this coloring method may turn to be too long. However since the MBSZones are always planned in advance, it may be possible to avoidcreating too complex topologies.

The entity which calculates the graph coloring, should be made aware ofthe data accumulated for each MBS Zone at the end of each accumulationperiod. Such an entity can be a Sync Node shared by multiple Zones (seeFIG. 11).

It should be noted that it is possible to have a separate Sync Node pereach Zone (or per a number of zones being a sub-set of the total numberof zone handled). In this case, at the end of each accumulation periodthe Sync Nodes must exchange information about the amount of trafficaccumulated for each Zone. The Sync Node of each such specific zone canthen calculate the graph coloring independently and send the assignmentof the resulting colored region to its Zone.

As will be appreciated by those skilled in the art, the examplesprovided show methods and devices for synchronizing multicast/broadcastservices in wireless networks. However, similar processes may be appliedin a similar way in order to accommodate different network'sconfigurations, without departing from the scope of the presentinvention.

It is to be understood that the above description only includes someembodiments of the invention and serves for its illustration. Numerousother ways of carrying out the methods provided by the present inventionmay be devised by a person skilled in the art without departing from thescope of the invention, and are thus encompassed by the presentinvention.

1. In a wireless communications network comprising a plurality of BaseStations each operative to provide MBS to mobile subscribers, a methodfor synchronizing the transmission of multicasting/broadcasting services(MBS) data by said plurality of Base Stations comprising the steps of:(i) providing a first pre-determined time interval; (ii) providingsynchronization information to enable all of said plurality of BaseStations to start transmitting MBS data at a synchronized starting timepoint; at each of said plurality of Base Stations: (iii) receiving MBSdata to be transmitted by all of said plurality of Base Stations; (iv)determining whether any of the MBS data has not been properly received,and upon determining the existence of such faulty MBS data, initiatingby the respective one or more Base Stations a process to recover saidfaulty MBS data or to obtain within said first pre-determined timeinterval, information regarding the duration of a time period that wouldhave been required for transmission of the faulty MBS data; (v) for anyof the respective one or more Base Stations that has not recovered saidfaulty MBS data, determining a starting point of a silence period whichwould proceed the last data that has been properly received at therespective Base Station and the duration of said silence period based onthe information obtained by said respective Base Station; (vi)transmitting available MBS data, starting at the synchronized startingtime point and for the duration of a second pre-determined timeinterval, and wherein each of said respective one or more Base Stationsthat has not recovered the faulty MBS data shall refrain during saidsilence period from transmitting any signals along a communicationchannel allocated for transmission of MBS data.
 2. The method accordingto claim 1, wherein said first per-determined period of time comprisestwo substantially equal periods of time, the first of which is intendedfor carrying out step (iii) while the second for carrying out step (iv).3. The method according to claim 1, wherein the duration of said firstpre-determined period of time is twice as long as the secondpre-determined time interval.
 4. The method according to claim 1,wherein steps (iii) to (vi) are repeated every interval of time which isequal to the sum of the first pre-determined time interval and thesecond pre-determined time interval.
 5. The method according to claim 1,wherein steps (ii) to (vi) are repeated after receiving synchronizationinformation from a network scheduling means.
 6. The method according toclaim 1, wherein said synchronization information comprises at least oneof the following: global multicast flow identification for identifying arespective multicast flow; timestamp indicating the start of the firstpre-determined timeinterval; and sequence numbers of the first and lastpackets received during the first pre-determined timeinterval;scheduling rules; and resources allocations.
 7. The method according toclaim 1, wherein said synchronization information further comprises atleast one of the following: allocation of MBS bursts across air framesof a transmission period; order of bursts; information allowing saidplurality of Base Stations to construct identical PDUs.
 8. The methodaccording to claim 1, wherein the process in step (iv) to recover saidfaulty MBS data or to obtain information about the time period requiredfor transmission of said faulty MBS data, comprises the steps of: (a)sending by each of the respective one or more Base Stations acorresponding recovery request message to a distributing gateway whichcomprises identification of the respective multicast flow and a list ofsequence numbers of faulty packets; (b) receiving from said distributinggateway a recovery response message comprising identification of therespective multicast flow and a list of sequence numbers of the faultypackets together with information relating to their correspondinglengths.
 9. The method according to claim 8, wherein said recoveryresponse message further comprising information relating to the dataincluded in one or more of the faulty packets.
 10. The method accordingto claim 1, wherein step (vi) is synchronized with a paging period ofrespective mobile subscribers so that such mobile subscribers can avoidwaking up twice to receive MBS data and paged data during two differenttime intervals.
 11. The method according to claim 1, wherein saidwireless communications network comprises a plurality of MBS zones, eachcomprising at least one Base Station and wherein at least two of saidMBS zones geographically overlap each other, and wherein said methodfurther comprising a step of synchronizing all MBS zones that have nooverlapping between each other so that they may transmit their data overthe same subcarriers at the same time.
 12. The method according to claim11, wherein said step of synchronizing all MBS zones is carried outdynamically so that after each first pre-determined time interval periodthe network resources are re-assigned and distributed among all BaseStations that belong to the corresponding MBS zones.
 13. A base stationoperative to provide multicasting/broadcasting services (MBS) to mobilesubscribers in a wireless communications network, said wirelesscommunications network comprising a plurality of base stations eachoperative to provide MBS to mobile subscribers, wherein said basestation comprises: a transceiver operative to: (i) receivesynchronization information to enable said base station to starttransmitting MBS data at a synchronized starting time point; (ii)receive MBS data to be transmitted by all of said plurality of basestations; a processor operative to: (i) determine whether any of the MBSdata has not been properly received, and upon determining the existenceof such faulty MBS data, initiating a process to recover said faulty MBSdata or to obtain within a first pre-determined time intervalinformation regarding the duration of a time period that would have beenrequired for transmission of the faulty MBS data; (ii) in case that saidfaulty MBS data has not been recovered, to determine a starting point ofa silence period which would proceed the last data that has beenproperly received and the duration of said silence period based on theinformation received by said transceiver; and wherein said transceiveris further operative to transmit available MBS data, starting at thesynchronized starting time point and for the duration of a secondpre-determined time interval, and wherein in case the faulty MBS datahas not recovered, to refrain from transmitting any signals along acommunication channel allocated for transmission of MBS data during saidsilence period.
 14. The base station according to claim 13, wherein saidsynchronization information comprises at least one of the following:global multicast flow identification for identifying a respectivemulticast flow; timestamp indicating the start of a time interval duringwhich MBS data shall be received by said plurality of Base Stations andeach of said Base Stations may conduct a recovery process in case someor all of the MBS data received thereby is missing; and sequence numbersof the first and last packets received during said time interval. 15.The base station according to claim 13, wherein said synchronizationinformation further comprises information that enables coherentconstruction and placement of bursts.